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PIITIIAUC ACID                         217
sublimes readily, and can be distilled in steam.    It is soluble in
most of the common organic solvents.
Rear/ion.ŚMake strong solutions of about equivalent quanti-
ties of naphthalene and picric acid in acetic acid, or alcohol, and
pour them together. On cooling, yellow, needle-shaped crystals
of naphthalene picrate separate : C1()H8-fCGH2(NO^)3OH ; m. p.
Phthalic Acid, CfiH./cS;g8   \
Friedlandcr, TJiccrfnrbcnftibrikntion^ iv, 164,
15 grins, naphthalene.
120 c.c. cone, sulphuric acid.
7'5 grins, mercuric sulphate.
The mixture of naphthalene, sulphuric acid, and mercuric
sulphate is placed in a retort (300 c.c.). The retort is clamped
\vilh the neck sloping upwards, and heated gently over wire-
gauze with occasional shaking until the liquid surface layer
of naphthalene dissolves.* The retort is now placed in the
ordinary position, with the neck sloping down, to which a con-
denser tube is attached by means of a roll of asbestos paper, or
a lute of plaster of Paris. The end of the condenser tube is
provided with a receiver containing water (too c.c.), and cooled
in cold water.
The retort is now heated (at first cautiously and then strongly)
over the bare flame, and the contents distilled. The liquid
rapidly darkens in colour. At about 250' oxidation begins, with.
evolution of sulphur dioxide, which becomes very vigorous as the
temperature of the liquid rises to the boiling-point. A little
naphthalene first distils, and after a time crystals of phthalic
anhydride appear in the condenser tube, whilst phthalic acid
collects in the receiver. The distillation is continued until the
residue becomes viscid or even dry. The contents of the
receiver, when cold, are filtered and washed, and then dissolved
in caustic soda. Any undissolved naphthalene is removed by
filtration, and the acid rcprecipatccl by hydrochloric acid. The